I don't know today but definitely in the 20th century the correct spelling was foolscap paper. As a teenager I dabbled in a stationery supply business for a year and that's how it was spelt on the cartons that came from the paper mills. It is a Commonwealth origin spelling . Foolscap is paper cut to the size of 8 1⁄2 ◊ 13 1⁄2 inches. This was a traditional paper size used in Europe and the British Commonwealth, before the adoption of the international standard A4 paper, the most common standard size in the world today.